This invention relates generally to the field of radio frequency (RF) fixture devices, and more specifically to a method of controlling electronics across an RF barrier.
RF enclosures may be used in a variety of test and measurement applications when the amount of RF radiation that impacts a device under test (DUT) must be carefully controlled. Specific examples include prototype testing and production testing of cellular telephones, portable computers, pagers, and other small electronic devices. Often RF enclosures are used in automated or semi-automated environments in which machines are used to place the DUT inside the RF enclosure. In these types of test environments, it is desirable to be able to evaluate the functionality and correct operation of the DUT while the DUT is within the RF enclosure. The use of electronics circuitry and software may be used to apply specific test resources to the DUT and measure the responses. In most applications, the electronics circuitry and software is located both internal and external to the RF enclosure. However, placing high speed electronics within the RF enclosure reduces the RF isolation and generates spurious noise, therefore degrading the testing accuracy. The proximity of the electronics to the DUT also influences the measurement accuracy. Thus, in test and measurement situations involving an RF enclosure, there are often two competing design considerations:
Including more sophisticated electronic components within an RF enclosure allows more accurate and more comprehensive test procedures to be executed. However from the point of view of RF isolation and noise reduction, fewer electronics components create smaller amounts of spurious RF energy that leads to improved RF measurements. These competing considerations must both be addressed when designing RF test and measurement systems that incorporate RF enclosures.
A further issue when electronics components are used within an RF enclosure is the complexity of the interface between the electronics inside the RF enclosure and the electronics outside the RF enclosure. As the number of communication paths that cross the RF interface increase, so does the amount of RF noise generated by each path. Thus, a secondary design consideration is the method of controlling the electronics path across an RF barrier of an RF enclosure.
There are many different methods of controlling electronics across an RF barrier of an RF enclosure. The RF enclosure may be used to test and prototype electronics devices, such as cell phones, personal digital assistants (PDAs), and other similar electronics. Most methods of controlling electronics across an RF barrier use electrical signals propagating through a filtered connector as the primary transport mechanism. These electrical signals can carry data reads and data writes to and from the electronics within an RF enclosure. Most of these methods may be classified into one of three basic types: direct drive, wide parallel, and command serial.
A direct drive control method consists of providing dedicated electrical resources outside of the RF enclosure that pass through the filtered connector(s) for each element being controlled. An example of this type of control method would be to generate relay coil drive signals outside the RF cavity and pass each of these drive lines through the filtered connectors to relays inside the cavity. This method requires an individual control line that passes through the filtered connector for each resource being controlled. It is desirable to limit the number of control lines passing through the filtered connectors since these filtered connectors are very expensive (approximately $2.00/line) and the effective RF isolation of the structure degrades with the number of signals passing through the filtered connectors. A direct drive scheme is also not very flexible or expandable.
A wide parallel control method consists of a conventional address, data, and control scheme in which all signals must pass through the filtered connectors(s) to register based electronics inside the RF cavity. This method often requires a large number of signals passing through the filtered connectors. A typical implementation might consist of 16 or more lines: 8 data, 5 address, and 3 control. Many RF test fixtures such as the TS-50 (Yukon) and the Z2030A (Osprey) use this control method.
A command serial control method consists of sending commands across the filtered connector(s) to a microprocessor or other programmable logic device that can interpret these commands into control sequences. This method commonly requires a clock to be running at all times inside the RF enclosed portion for clocking the microprocessor or logic device. This clock creates spurious noise that can interfere with the measurements inside the RF cavity. Command serial control methods are also more complicated and often require some firmware programming.
Other methods of control across an RF barrier exist that do not use an electrical connection. An example of this is an optical link. These methods tend to be more expensive and require a free running clock that adds to the electrical noise inside the RF cavity.
Thus, there is an unmet need in the art for a simple, cost effective method of controlling electronics across an RF barrier. Such a method should provide an ideal environment for test RF devices, have good RF isolation, encompass a less expensive design with fewer lines and built-in interfaces, have low spurious noise, be simple to implement, and be flexible and extensible to future enhancements.
The present invention takes advantage of the industry standard serial data control buses capable of serially shifting data in and/or out of electronics being controlled in some pre-determined protocol to provide a serial data control protocol for controlling electronics across an RF barrier. A microprocessor with a built-in serial data control bus (e.g., SPI) exists outside of the RF chamber. An address dependent form of the serial bus passes through one or more RF filtered connectors into the RF cavity. The processor uses the serial data bus to control (data reads and data writes) an arbitrary amount of electronics inside and outside of the RF cavity. This control method requires a minimal number of signals passing through the filtered connectors, preserving the RF isolation of the cavity from the external environment.
The addressing scheme is designed to specifically isolate electronics outside the RF chamber from the electronics inside the chamber. The serial data bus lines that pass through into the RF cavity are held in a quiescent state unless being used to control electronics inside the RF cavity. This signal isolation technique provides a very low spurious noise environment inside the RF cavity. This is important for sensitive RF measurements, while allowing the processor to control external electronics. This gating functionality may additionally have a temporal aspect that controls when individual signals are gated from entering the RF cavity. This provides the important advantage of being able to selectively control noise levels experienced during the testing process.